1. Field of the Invention
The present invention relates to an optical information recording medium that can record/reproduce high quality information signals by irradiating a thin film formed on a substrate with a high energy beam such as a laser beam. The present invention also relates to a manufacturing method and a recording/reproducing method for the optical information recording medium.
2. Description of the Related Art
It is widely known that a thin film made of a chalcogen material or the like is formed on a substrate and irradiated with a laser beam for local heating so as to cause a phase change between an amorphous phase and a crystalline phase depending on the irradiation conditions. The amorphous phase and the crystalline phase differ in optical constants (refractive index n and extinction coefficient k). The research and development of a so-called phase change optical information recording medium, which uses such a chalcogen thin film as a recording film by taking advantage of the phase change phenomenon, has been conducted actively in recent years.
In the phase change optical information recording medium, a new signal can be recorded while erasing the existing signal, e.g., by irradiating an information track with a laser beam that is modulated between at least two power levels of a recording level and an erasing level in accordance with the information signal.
This phase change optical information recording medium generally has a multi-film laminating structure that includes a recording film and additional films other than the recording film. In many cases, the additional films include, e.g., a protective film made of a dielectric material, a reflective film made of a metal or alloy material, etc.
The protective film has, e.g., the following functions of:
1) protecting the recording film from external mechanical damage;
2) reducing thermal damage caused by repetitions of rewriting signals, such as the thermal deformation of a substrate surface, the defects of the recording film, and the evaporation of components of the recording film, thus increasing the number of rewriting operations;
3) enhancing an optical change by utilizing the interference effect of multiple reflection; and
4) preventing a chemical change of the recording film by blocking off the outside air.
To achieve the above functions of the protective film, e.g., an oxide such as Al2O3, TiO2, and SiO2, a nitride such as Si3N4 and AlN, an oxynitride such as Si—O—N, a sulfide such as ZnS, a carbide such as SiC, or a mixture of these compounds such as ZnS—SiO2 have been proposed and used as a material for the protective film. ZnS—SiO2 is most widely used for the following reasons. Particularly, the thermal conductivity of ZnS—SiO2 is considerably lower than that of the other dielectric materials. Therefore, ZnS—SiO2 sufficiently can suppress thermal diffusion that occurs when a laser beam or the like is used for recording, and improve the recording sensitivity. A ZnS—SiO2 film causes fewer defects even if it has a large thickness because of its small internal stress. Moreover, even if the laser beam irradiation is repeated, the film does not peel off easily because of its excellent adhesion to the recording film made of a phase change material.
An interface film provided between the recording film and the protective film has been proposed. The interface film has, e.g., the following functions of:
1) accelerating the crystallization of the recording film to improve the erasing characteristics; and
2) preventing the interdiffusion of substances between the recording film and the protective film to improve the durability for repetitive recording.
It is more desirable that the interface film also has environmental reliability such that it is not corroded and peeled off from the recording film.
As a material for the interface film, e.g., a nitride of Si or Ge is much superior in the effects of accelerating the crystallization of the recording film and preventing the diffusion of the substances (disclosed, e.g., by JP 5(1993)-217211 A and WO 97/34298). Particularly, an interface film that includes Ge—N as the main component and Cr or the like does not peel off easily from the recording film even under high temperature and high humidity. Therefore, this material is one of the most suitable materials for the interface film.
The amount of information to be stored in the above optical information recording medium can be increased basically when the spot diameter of a laser beam is reduced to improve the areal recording density. The spot diameter can be reduced by shortening the wavelength of a laser beam or increasing the numerical aperture of an objective lens for focusing the laser beam. In recent years, it has been proposed that a blue laser diode having a wavelength of about 400 nm, which is proceeding toward practical use, is used in an optical system for recording/reproducing information with respect to an optical information recording medium, and that the numerical aperture of an objective lens of the optical system is increased, e.g., from 0.60 for DVD-RAM (DVD-Random Access Memory) etc. to about 0.85, thereby reducing the laser spot diameter to improve the areal recording density. However, an increase in numerical aperture causes a decrease in tolerance for tilt of the optical information recording medium. Therefore, it also has been proposed that the thickness of a transparent substrate located on the laser beam incident side is reduced from 0.6 mm for DVD-RAM etc. to about 0.1 mm.
To increase the amount of information available for a single optical information recording medium, an optical information recording medium having a multilayer structure (referred to as “multilayer recording medium” in the following of this specification) has been proposed as well. The multilayer recording medium includes a plurality of layers for recording/reproducing information (referred to as “information layer” in the following of this specification). In the multilayer recording medium, a laser beam is absorbed by an information layer located closer to the laser source, so that the attenuated laser beam is used to record/reproduce information with respect to an information layer located farther from the laser source. This causes the problems of reduction in sensitivity for recording and in reflectance and amplitude for reproduction. Therefore, the information layer located closer to the laser source should have a higher transmittance, while the information layer located farther from the light source should have a higher reflectance, reflectance difference (i.e., a difference in reflectance between the crystalline phase and the amorphous phase of the recording film), and sensitivity so as to achieve sufficient recording/reproducing characteristics with limited laser power.
The use of a blue laser and an objective lens having a large numerical aperture increases the energy density of the laser beam. Therefore, the recording film in the information layer absorbs the laser beam during reproduction, and a portion of a recording mark disappears more easily. This phenomenon is called “degradation caused by reproduction light.” To solve this problem, the light absorption of the recording film can be reduced by allowing any film in the information layer other than the recording film (e.g., the interface film) to absorb light appropriately, thus suppressing the degradation caused by reproduction light, i.e., improving the durability against reproduction light.
However, this method leads to a reduction in reflectance change and lowers the signal quality such as a C/N ratio.